The following exemplifies conventional reflective materials capable of highly reflecting light and conventional structures made of highly reflective materials.
(1) High Q optical resonator using dielectric sphere: Generally, a dielectric micro sphere having a micrometer size is used. Light circularly travels along an inner circumference of the dielectric micro sphere while being totally internally reflected. Therefore, a remarkably high Q optical resonator can be obtained though no reflecting mirror is provided. Such a remarkably high Q optical resonator is called Whispering Gallery Mode, and frequently used in scientific experiments. For example, such dielectric micro spheres are provided for use in a slow-wave circuit. Alternatively, such a dielectric micro sphere whose inside is doped with a pigment is used for a microsphere laser.
(2) Mirror ball: A sphere whose top surface is tightly covered with pieces of mirror, or an entertainment lighting apparatus that rotates a polyhedron by use of a motor. Millar ball irradiated with a spotlight dispersedly emits polka-dot light throughout a room, so as to create an air of fantasy.
(3) Natural or artificial jewel: A natural or artificial jewel has a remarkably high refractive index. Light having entered the natural or artificial jewel is easily totally internally reflected. Particularly, the natural or artificial jewel cut so as to have a polyhedral surface increases its brightness.
Patent Literature 1 discloses a lighting apparatus which is used as a front reflecting mirror by providing arrayed corner cubes around a light source.
Other examples of the structures made of highly reflective materials encompass optical structures such as a light guide and an optical resonator each made of an optical medium.
(4) Light guide (Waveguide): Most of light guides (waveguides) typified by optical fibers and made of a transparent optical medium are arranged such that a core having a high refractive index is provided at a center, and a clad having a low refractive index is provided around the core. Light from the core is totally internally reflected when coming into contact with the clad. The light is confined in the core without being radiated to an outside of the core, and travels in the core. Such a guided mode is employed for transmission. Meanwhile, light having (i) a wide intersection angle when coming into contact with the clad (having a narrow entrance angle) and (ii) a wide spread angle which prevents the light from being totally internally reflected leaks to the outside of the core. This results in a radiation mode having a great transmission loss. Further, when we use light guides of this type for transmission with high optical power, the light guides need to have a great aperture. This causes problems of a heavy weight and a light loss in the core.
Another example of the optical structures is a hollow light guide which is hollow and has an outer surface on which an optical medium is provided. The hollow light guide is light even in case where the hollow light guide has a great aperture. The hollow light guide is suitable for transmission with high optical power since the hollow light guide has a hollow core. The hollow light guide is further devised by, for example, employing a guided mode in which no light is radiated to an outside of the hollow core, or providing the outer surface with a metal film so as to reflect light.
In the above cases, there are two modes: (i) the guided mode in which optical transmission is carried out with less loss, and (ii) the radiation mode in which light is radiated to an outside, so that a transmission loss is increased. Normally, only the guided mode is employed. Note that, in a case where the light guide is connected with an external apparatus or another transmission line, perfect mode matching is difficult. Therefore, the radiation mode partially exists and causes a coupling loss. A hollow light guide with a metal film can suppress the radiation mode but a problem of an absorption loss in the metal film is caused.
The guided mode commonly corresponds to light beams which have a narrow spread angle and do not travel at a wide angle with respect to a waveguide axis direction. Meanwhile, the radiation mode corresponds to light beams which have a wide spread angle and have a component that travels at a wide angle with respect to the waveguide axis direction. In the radiation mode, the light beams leak to an outside without being subjected to total internal reflection. Total internal reflection requires an entrance angle wider than a critical angle determined depending on a refractive index difference. Therefore, from the viewpoint of light rays, a mode, which corresponds to light that travels in a zigzag manner principally in an axis direction and slightly in a radial direction, is the guided mode. A mode, which corresponds to light that greatly meanders more in the radial direction than in the axis direction and has great difficulty in traveling forward, is the radiation mode.
A hollow light guide that has recently gained attention is exemplified by a photonic crystal fiber. The photonic crystal fiber has many advantages, whereas the photonic crystal fiber has disadvantages of a complicated structure and limitation on an operating wavelength region due to a periodic structure substantially equivalent to a wavelength.
In view of the circumstances, for example, Patent Literature 2 discloses a hollow light guide as a hollow light guide that does not have a complicated structure unlike the photonic crystal fiber. The hollow light guide disclosed in Patent Literature 2 guides light by causing its wall to totally internally reflect the light.
According to the hollow light guide disclosed in Patent Literature 2, at least one triangular ridge is provided on an outer surface of a square tube in an axial direction of the square tube, so as to guide light by causing a wall of the hollow light guide to totally internally reflect the light. It is therefore necessary to transmit light beams having an angle falling within an angle determined depending on a refractive index. The hollow light guide disclosed in Patent Literature 2 serves as a transmission line for transmitting light while preventing light beams from spreading.
That is, the hollow waveguide disclosed in Patent Literature 2 has the so-called radiation mode in which light beams having an angle beyond the angle determined depending on the refractive index leak to an outside of the hollow waveguide in a case where the light beams are transmitted (see, for example, FIG. 42). It goes without saying that unnecessary light enters the hollow waveguide also from the outside of the hollow waveguide.
(5) Optical resonator: A typical example of a conventional optical resonator is Fabry-Perot resonator in which two mirrors face each other. The Fabry-Perot resonator confines only light along an optical axis, and disperses light other than the light along the optical axis. A semiconductor laser employs a waveguide structure, in which the radiation mode exists. Recently, a high Q resonator using a photonic crystal has been researched. Note, however, that, since a periodic structure which is substantially equivalent to a wavelength is employed in a photonic crystal as described above, the high Q resonator is limited in usable wavelength region. Depending on a case, the high Q resonator is also limited in direction in which light is emitted in the high Q resonator.